The present invention relates to a shutter control circuit used for cameras and more particularly to such a shutter circuit for cutting off a beam of light by opening and closing a shutter at different rates of speed to expose film in response to photo current developed by a photo sensitive element such as CdS.
One recent programed shutter mechanism including blades functions as an iris where, before release of a shutter switch, the blades completely close an aperture of a camera and, after release, the iris is opened to control the amount of light reaching the film of the camera. Before the iris is completely opened, there is a delay of about 0 to 60 msec.
FIG. 1 shows a graph representing relationship between area (S) of an opened aperture by the shutter blades and time passage (T). The shutter blades are completely opened after time passage T.sub.O. If a rapid shutter speed is set in the camera, the shutter becomes closed prior to complete opening by the iris. If a slow shutter speed is set in the camera, the shutter becomes closed after the iris has been completely open. Refer to condition a in FIG. 1 in which a rapid shutter speed is set, condition b in which a medium shutter speed is set, and c condition in which a slow shutter speed is set.
As is apparent from the graph of FIG. 1, the values of the areas S continue to increase prior to a time T.sub.O while the values of the areas S are kept constant after the time T.sub.O.
Therefore, it is necessary to utilize different ways to obtain an appropriate amount of exposure depending on whether time a is necessary before or after the time T.sub.O. Suitable careful attention is not directed to the above described characteristics of the exposure by the iris diaphragm in the conventional shutter mechanism.